1. Field of the Invention
The present invention relates generally to a direct injection internal combustion engine, and more particularly to a method of warming up catalysts when the direct injection internal combustion engine is started cold.
2. Description of Related Art
In recent years, a direct injection internal combustion engine, which injects fuel directly into a combustion chamber, has been developed so as to improve both engine output and fuel economy. In such an internal combustion engine, it is desirable to quickly activate catalysts disposed in an exhaust passage and reduce the amount of harmful substances such as unburned HC that is emitted into the air.
For example, an intake port injection internal combustion engine ordinarily retards the ignition timing to a point after a top dead center in order to accelerate the warm-up of the catalysts. For this reason, it can be considered that this method is applied to the direct injection internal combustion engine. In this case, the ignition timing is set at a point after the top dead center in an intake stroke injection because the intake port injection internal combustion engine ordinarily performs a uniform pre-mixed combustion.
The above internal combustion engine, however, is greatly affected by the residual gases due to the decrease in the intake air when the load on the engine is high e.g. while idling. Thus, a combustion reaction cannot be satisfactorily carried out in the pre-mixed combustion caused by the intake stroke injection, and the retardation of the ignition timing as shown in FIG. 10(c) results in unstable combustion. Consequently, as shown in FIGS. 10(a) and 10(b), the combustion variation ratio is increased to make the heat release amount uneven and further, the engine may misfire. The misfire of the engine causes generation of the unburned HC and deterioration of the exhaust gas performance.
To meet the above-mentioned requirement, there has been developed a method (hereinafter referred to as a two-stage combustion) in which a main injection and an additional fuel injection (hereinafter referred to as an additional injection) are performed by making use of such a characteristic that the direct injection internal combustion engine is capable of freely setting a fuel injection timing. In this method, the additional fuel is burned so as to raise the temperature of exhaust gases by the operation of a reaction product resulting from a main combustion caused by the main injection. This contributes to the quick activation of the catalysts.
Since the combustion reaction of the additional fuel is a low-temperature oxidizing reaction that proceeds slowly, however, a part of the fuel injected in the additional injection is exhausted into an exhaust pipe before burning up in the combustion chamber. This necessitates continuing the combustion reaction of the residual fuel in the exhaust pipe in order to reduce the generation of the unburned HC.
To solve this problem, Japanese Patent Provisional Publication No. 11-294157 discloses a direct injection internal combustion engine that has exhaust manifolds having a capacity space for holding exhaust gases and burns the residual fuel in the capacity space of the exhaust manifolds in order to reduce the generation of unburned HC and raise the temperature of the exhaust gases. Although the method disclosed in Japanese Patent Provisional Publication No. 11-294157 enables the quick activation of the catalysts and the reduction in the unburned HC emission, an output from the engine is lowered because it is interfered in the capacity space by the exhaust gases between cylinders. This method is therefore unsuitable for an engine that is required to output high power.
Another method has been proposed which integrates a proximity catalyst with exhaust manifolds so that the proximity catalyst can be provided at a low cost. In this case, it is difficult to ensure a sufficient capacity space in front of a proximity catalyst. For this reason, the use of the exhaust manifolds integrated with the proximity catalyst greatly makes it difficult to adopt the method disclosed in Japanese Patent Provisional Publication No. 11-294157.
Further, in the two-stage combustion, almost all of the fuel injected in the additional injection transforms into heat and thus hardly contributes to the engine output. Accordingly, the fuel injected in the main injection must be increased and the fuel injected in the additional injection must be decreased accordingly as shown in FIG. 11(a) under such a condition that the load on the engine is high (e.g. while the engine is idling with a shift position of an automatic transmission being set in a D range, or while the engine is running an accessory such as an alternator and an air compressor). The decrease in the fuel injected in the additional injection makes it impossible to satisfactorily increase the temperature of the exhaust gases and accelerate the activation of the catalysts as shown in FIG. 11(b), and also deteriorates the exhaust gas performance as shown in FIG. 11(c). This imposes a restriction on the driving conditions that enable the additional injection to accelerate the warm-up of the catalysts.
As stated above, the method in which the catalysts are warmed up and activated by performing the additional injection as well as the main injection is greatly affected by the shape of the exhaust manifolds, the driving conditions and the like. Therefore, the satisfactory effects may not be achieved due to the shape of the exhaust manifolds, the driving conditions and the like.
The present invention to provide a direct injection internal combustion engine that is able to rapidly warm up and quickly activate catalysts in a more efficient manner. In a preferred embodiment, the present invention provides a direct injection internal combustion engine comprising: catalysts for purifying exhaust gases, the catalysts being disposed in an exhaust passage of the engine; a control device for controlling the engine so as to warm up or activate the catalysts when the catalysts are required to be warmed up or activated; wherein the control device includes a first control part for controlling the engine with an air-fuel ratio of the engine being set at a value in proximity to a stoichiometric air-fuel ratio, an ignition timing being set after a top dead center and a fuel injection timing being set within a compression stroke; and a second control part for controlling the engine with an air-fuel ratio of the engine being set at a value in proximity to a stoichiometric air-fuel ratio, with an ignition timing being set at a point before the top dead center and a fuel injection timing being set within a compression stroke after the first control part controls the engine.